1. Technical Field
The present invention is directed to the art of phase lock loop control systems for controlling the output frequency of a voltage controlled oscillator and, more particularly, to improvements in such a system.
2. Description of the Prior Art
Phase lock loop controlled oscillators are widely used in RF transmission applications for both radio and television broadcast applications. It is important that the output frequency of such an oscillator be maintained relatively constant during operation. It has been known in the art to synchronize the operation of such an oscillator with a global positioning system (GPS). Also, the oscillator frequency should be maintained constant even though the GPS system is temporarily down and the oscillator is in what is known as a flywheel condition. The output frequency of the oscillator should be maintained within a predefined frequency error margin. The flywheel time may be on the order of 30 minutes to 24 hours and the output frequency of the oscillator should be maintained within the frequency error margin desired. The output frequency of an oscillator for RF operation may be on the order of 10 MHz. The permitted frequency deviation may be on the order of less than 0.01 Hz. The output frequency of such an oscillator may vary because of such factors as variations in ambient temperature and supply voltage.
The oscillators involved may frequently employ an oven-based temperature control crystal oscillator (OCXO) device. Such oscillators have been synchronized with GPS time as is indicated by various patents in the prior art. These patents include, for example, the U.S. patent to Hitch et al. U.S. Pat. No. 5,861,842 and the U.S. patent application publication number U.S. 2007/0035345A1.
It is known to control such oscillators by employing phase lock loop controllers (which may include a proportional integral and differential controller, known as a PI or PID controller). Controllers that employ a fixed proportional gain or a fixed integral time constant may have difficulty performing the operation within the desired requirements for RF frequency operation.